This is a prospective, open-label pharmacokinetic interaction study of the addition of efavirenz to a four-drug antiretroviral regimen containing 2 nucleoside analogs, ritonavir, and saquinavir. 16 HIV-infected, asymptomatic subjects on a stable (two weeks) regimen of any two nucleoside analogs and ritonavir (400 mg BID) and saquinavir (400 mg TID) with Viral RNA below the limits of detection of first generation assays are being studied. Efavirenz, a non-nucleoside transcriptase inhibitor, is added to the patients current regimen followed later by doubling the saquinavir. Three 24 hour PK periods are involved, one for each of the 3 regimens.